As mentioned in a comment by Drew Royals below, Liturgicam Authenticam legislates a kind of conservatism regarding hymn texts in #108.
Sung texts and liturgical hymns have a particular importance and efficacy. Especially on Sunday, the “Day of the Lord”, the singing of the faithful gathered for the celebration of Holy Mass, no less than the prayers, the readings and the homily, express in an authentic way the message of the Liturgy while fostering a sense of common faith and communion in charity. If they are used widely by the faithful, they should remain relatively fixed so that confusion among the people may be avoided…
This point seems carefully worded. The legislation does not say that the texts must be absolutely fixed, but relatively fixed. There might be compelling reasons to change the wording of a hymn. Generally speaking, however, it is pastorally more sensitive to keep the wording of hymns steady over time.